On some of my stouts/porters, I add my dark/black malts just before I start batch sparging. So, I get all the color and flavor, and none of the bitterness. Tis a good technique to avoid any potential bitterness from mashing the black malts, since they don't really need to be mashed.

I brewed a Black IPA where I mashed without the dark malt, then mixed it in during the (batch) sparge. It turned out perfectly: black as a stout with no roast character at all. Granted, I can't tell if that's because of the malt (Carafa II Special dehusked), or because I mashed without it and added it during the sparge, but you can't argue with results.